Section 1: Earth’s Crust in MotionHow Do Stress Forces Affect Rock?

• The movement of earth’s plates creates powerful forces that squeeze or pull the rock in the crust – these forces are examples of stress

• Stress – a force that acts on rock to change its shape or volume

• An earthquake is the shaking and trembling that results from the movement of rock beneath earth’s surface

How Does Stress Affect the Earth’s Crust?

• Deformation – any change in the volume or shape or earth’s crust

• Three kinds of stress in the crust:

–Shearing – stress that pushes a mass of rock in two opposite directions

–Tension – pulls on the crust, stretching rock so that it becomes thinner in the middle like warm bubble gum

–Compression – squeezes rock until it folds or breaks like a giant trash compactor

What Is a Fault?

•Fault – a break

in earth’s crust where slabs of crust slip past each other; These usually occur at plate boundaries

a. Most faults lie between the surface and a depth of 70 kilometers

b. Focus- point below Earth’s surface where rocks break and move

c. Epicenter- point above Earth’s surface directly above the focus

What Kind of Faults Are There?

• Three Kinds:

–Normal Faults–Reverse Faults–Strike-slip faults

What Are Strike-slip Faults?

• Strike-slip faults

–Shearing forces cause

rocks to slip past each other

sideways with little up

and down Motion;

– Ex. San Andreas fault in California

What Are Normal Faults?

• Normal faults

–Tension forces cause the rocks to form

the fault at an angle– One block is above the fault

–Hanging wall – the half of the fault that lies above

–Footwall – the half of the fault that lies below

– Ex. Rio Grande rift valley

Normal Fault

What Are Reverse Faults?

• Reverse faults

–compression forces cause the rocks to move

towards each other

– Same structure as normal fault but the blocks move in opposite direction; hanging wall move up

– Ex. Appalachian Mountains and Mount Gould in Glacier National Park

Reverse Fault

What type of fault?What type of fault?

A miner walks on the foot wall and looks up at the hanging wall!

A B

Normal Fault Reverse Fault

Hanging wall moves down

Hanging wall moves up

Section 2: Measuring QuakesHow Does the Energy of an Earthquake Travel Through Earth?

• Earthquakes – most begin in the lithosphere

• Focus – the point beneath the earth’s surface where rock that is under stress breaks, triggering an earthquake

• Epicenter – the point on the earth’s surface directly above the focus

What Are Seismic Waves?

• Seismic Waves – vibrations that travel through Earth carrying the energy released during an earthquake – They move like ripples on a pond– They carry the energy of an earthquake

away from the focus, through Earth’s interior, and across the surface

– The energy is greatest the the

Epicenter

What Are the Different Kinds of Seismic Waves?

• Three categories:

–P waves–S waves –Surface waves

• P waves and S waves are sent out from

the focus; Surface waves develop when the waves reach the

surface

What Are P Waves?

• P waves are primary waves

– The first waves to arrive

– Earthquake waves that

compress and expand the ground like an accordion

– Cause buildings to contract and expand

What Are S Waves?

• S waves are secondary waves– Earthquake waves that vibrate from side

to side as well as up and down– These waves shake the ground

back and forth– Shake structures violently

–Cannot move through liquids

What Are Surface Waves?

• When P waves and S waves reach the surface some are transformed into surface waves– Surface waves move more

slowly than P waves and S waves

–Produce the most severe ground movements

– Can make the ground roll like ocean waves or shake buildings from side to side

How Do Scientists Detect Seismic Waves?

• Seismograph – records

the ground movements caused by seismic waves as they move through the Earth

                  

How Do Scientists Measure Earthquakes?

• There are at least 20 different measures for rating earthquakes, three are:

–Mercalli–Richter–Moment Magnitude

•Magnitude – a measurement of earthquake strength based on seismic waves

What Is the Mercalli Scale?

• Rated earthquakes according to

their intensity

–Intensity: strength of ground motion in a given place

• Not a precise measurement• Describes how earthquakes

affect people, buildings, and the land surface

What Is the Richter Scale?

• A rating of the size of

seismic waves as measured by a particular type of seismograph

• Accurate measurements for small, nearby earthquakes not large, distant earthquakes

What Is the Moment Magnitude?

• A rating system that

estimates the total energy released by an earthquake

• Can be used to rate earthquakes of all sizes, near or far

• Below 5.0 – little damage

• Above 5.0 – great destruction

Earthquake damage in Charleston

February 21, 1916 Asheville, NC - the most intense earthquake in NC history, measuring a 6 on the Mercalli scale.

June 5, 1998 an earthquake in Moorseville, NC measured 3.2 on the Richter scale, but there was no reported damage.

Mantle- directly above the outer core

1. Plasticity is the property of a solid with the ability to flow like a liquid at very, very slow rates.

a. High temperature and pressure allow the rock to flow like a liquid

b. This also allows the plates of Earth (lithosphere) to move on the mantle.

MANTLE

The mantle is composed of silicon, oxygen, iron and magnesium.

3. Moho- boundary between the crust and the mantle.

a. Change in the speed of seismic waves moving through the Earth led to its discovery.

b. Discovered in 1909 by a Yugoslav scientist, Andrija Mohorovicic.

How Do Scientists Locate the Epicenter?

• Geologists use seismic waves– P waves arrive first– S waves arrive close behind

– Scientist measure the

difference in arrival times • The farther away an earthquake is the

greater the time between their arrival– Scientists draw three circles using data

from seismographs set at different stations to see where they intersect – the epicenter